Gas turbine manufacturers, including General Electric, are currently involved in research and engineering programs to produce new gas turbines that will operate at high efficiency without producing undesirable air polluting emissions. The primary air polluting emissions usually produced by gas turbines burning conventional hydrocarbon fuels are oxides of nitrogen, carbon monoxide and unburned hydrocarbons. It is well known in the art that oxidation of molecular nitrogen in air breathing engines is highly dependent upon the maximum hot gas temperature in the combustion system reaction zone and the residence time for the reactants at the highest temperatures reached within the combustor. The level of thermal NOx formation is minimized by maintaining the reaction zone temperature below the level at which thermal NOx is formed or by maintaining an extremely short residence time at high temperature such that there is insufficient time for the NOx formation reactions to progress.
One preferred method of controlling the temperature of the reaction zone of a heat engine combustor below the level at which thermal NOx is formed is to premix fuel and air to a lean mixture prior to combustion. U.S. Pat. No. 4,292,801 dated October 1981, the disclosure of which is hereby incorporated by reference, describes a dual stage-dual mode low NOx combustor for gas turbine application which is one of the pioneering combustor designs based on lean premixed combustion technology. U.S. Pat. No. 5,259,184 dated November 1993, the disclosure of which is also hereby incorporated by reference, describes a dry low NOx single stage dual mode combustor construction for a gas turbine. The thermal mass of the excess air present in the reaction zone of a lean premixed combustor absorbs heat and reduces the temperature rise of the products of combustion to a level where thermal NOx is not formed. Even with this technology, for the most advanced high efficiency heavy duty industrial gas turbines, the required temperature of the products of combustion at the combustor exit/first stage turbine inlet at maximum load is so high that the combustor must be operated with peak gas temperature in the reaction zone which exceeds the thermal NOx formation threshold temperature resulting in significant NOx formation even though the fuel and air are premixed lean. The problem to be solved is to obtain combustor exit temperatures high enough to operate the most advanced, high efficiency heavy duty industrial gas turbines at maximum load without forming a significant amount of thermal NOx.
Lean premixed combustion of hydrocarbon fuels in air is widely used throughout the gas turbine industry as a method of reducing air pollutant levels, in particular thermal NOx emissions levels, for gas turbine combustors. Lean direct injection (LDI) of hydrocarbon fuel and air has also been shown to be an effective method for reducing NOx emission levels for gas turbine combustion systems although not as effective as lean premixed combustion. An example of an LDI fuel injector assembly is described in an article from the 1987 Tokyo International Gas Turbine Congress entitled "Lean Primary Zones: Pressure Loss and Residence Time Influences on Combustion Performance and NOx Emissions," the disclosure of which is hereby incorporated by reference. The present invention combines these two technologies; i.e., lean premixed combustion and lean direct fuel injection, in a novel and unique manner in order to achieve extremely low air pollutant emissions levels, particularly oxides of nitrogen, when operating an advanced, high efficiency, heavy duty industrial gas turbine at high load.